1. Field of the Invention
The present invention relates to a Doppler frequency calculating apparatus and method that calculates a Doppler frequency, which is the magnitude of a time-dependent fluctuation of the characteristic of a transmission path for receiving an orthogonal frequency division multiplexing (OFDM) signal in motion, for example, and an OFDM demodulating apparatus that performs equalizing processing by using the calculated Doppler frequency.
2. Description of the Related Art
Digital signals may be transmitted by a modulating method called Orthogonal Frequency Division Multiplexing method (called OFDM method hereinafter). In OFDM method, data is assigned and is digitally modulated to the amplitude and phase of each of many orthogonal sub-carriers within a transmission band by PSK (Phase Shift Keying) or a QAM (Quadrature Amplitude Modulation).
In OFDM method, the total transmission speed is the same as that of a modulation method in the past though the modulation speed decreases since the transmission band is divided by the multiple sub-carriers and the band for each sub-carrier is narrow. Furthermore, in OFDM method, the symbol speed decreases since many sub-carriers are transmitted in parallel, and the relative time length of multipath can be shorter than the symbol time length, resulting in the resistance to multipath interference.
Still further, in OFDM method, since data is assigned to multiple sub-carriers, a transmitting/receiving circuit may include an IFFT (Inverse Fast Fourier Transform) calculating circuit that performs inverse Fourier transform upon modulation and an FFT (Fast Fourier Transform) calculating circuit that performs Fourier transform upon demodulation.
For these characteristics, OFDM method may often be applied in terrestrial digital broadcasting, which is susceptible to multipath interference. For example, the standards for the terrestrial digital broadcasting adopting such OFDM method may include DVB-T (Digital Video Broadcasting-Terrestrial) and ISDB-T (Integrated Services Digital Broadcasting-Terrestrial).
The transmission symbol in OFDM method (called OFDM symbol hereinafter) includes, as shown in FIG. 10, a valid symbol and a guard interval. The valid symbol is a signal period when IFFT is performed upon transmission. The guard interval is a copy of the waveform of a part of the second half of the valid symbol. The guard inertial is provided in the first half of the OFDM symbol. The existence of the guard interval in OFDM method allows the inter-symbol interference due to the multipath, which improves the multipath resistance.
For example, in Mode 3 of ISDB-TSB standard (which is a broadcasting standard adopted in Japan for terrestrial digital voice broadcasting. See “Chijo Dejitaru Onsei Hosoyo Jushin Souchi Hyoujun Kikaku (Nozomashii Shiyou) (Standard Specification (Desirable Standard) of Receiving Apparatus for Terrestrial Digital Voice Broadcasting) ARIB STD-B30”, Association of Radio Industries and Business, a valid symbol contains 512 sub-carriers, and the sub-carrier interval is 125/126≈0.992 kHz. In Mode 3 of the ISDB-TSB standard, 433 sub-carriers of 512 sub-carriers within a valid symbol are modulated by transmission data. In Mode 3 of ISDB-TSB standard, the time length of a guard interval is one of ¼, ⅛, 1/16 and 1/32 of the time length of the valid symbol.
OFDM method defines that one OFDM frame, which is a transmission unit, includes a collection of multiple OFDM symbols as described above. For example, in ISDB-TSB standard, one OFDM frame includes 204 OFDM symbols. In OFDM method, the position to which a pilot signal is to be inserted may be defined in the OFDM frame units, for example.
In OFDM method adopting QAM modulation as a modulation method for each sub-carrier, the characteristics of the amplitude and phase may differ among sub-carriers when the signals carried and modulated in the sub-carriers are distorted by the influence of the multipath upon transmission. Accordingly, received signals may need to be wave-equalized in a receiver side such that the amplitudes and phases of the sub-carriers can be equal. In OFDM method, a sender side scatters pilot signals with a predetermined amplitude and a predetermined phase within a transmission symbol in a transmitted signal while the receiver side monitors the amplitude and phase of the pilot signals, calculates the frequency characteristic of the transmission path, and equalizes the received signal by using the calculated characteristic of the transmission path. In OFDM method, a scattered pilot (SP) signal and/or a continual pilot (CP) signal, which will be described later, may be used as a pilot signal, which is to be used for calculating the characteristic of a transmission path.
FIG. 11 shows an arrangement pattern of SP signals within an OFDM symbol, which is adopted in ISDB-T standard.
In ISDB-T standard, one BPSK-modulated SP signal is inserted every 12 sub-carriers in the sub-carrier direction (frequency direction). Furthermore, in DVB-T standard or ISDB-T standard, the position where the SP signal is inserted is shifted in the frequency direction by three sub-carriers in each OFDM symbol. As a result, an SP signal is inserted every four OFDM symbols in one sub-carrier in the OFDM symbol direction (time direction)
In this way, in ISDB-T standard, an OFDM symbol is inserted with SP signals spatially scattered, whereby the redundancy of the SP signals is lowered for original information.
By the way, when the transmission path characteristic is calculated by using the SP signal, the characteristic can be specified for the sub-carrier to which SP signals are inserted. However, the characteristics may not be directly calculated for the other sub-carriers, that is, the other sub-carriers including original information. Therefore, the receiver side estimates the transmission path characteristics of the other sub-carriers including original information by filtering SP signals through a two-dimensional interpolation filter.
The processing of estimating a transmission path characteristic through a two-dimensional interpolation filter is generally performed as follows.
That is, in order to perform the processing of estimating a transmission path characteristic, the information component is eliminated from a received OFDM signal, and SP signals inserted at the positions shown in FIG. 11 are only extracted.
Next, the modulated component of each of the extracted SP signals is eliminated by using a reference SP signal. The SP signal from which the modulated component is eliminated exhibits the transmission path characteristic of the sub-carrier to which the SP signal is inserted.
Next, the SP signal from which the modulated component is removed is input to an interpolation filter in the time direction and undergoes time direction interpolation processing therein, whereby the transmission path characteristic of the sub-carrier including the SP signal is estimated for every OFDM symbol. As a result, as shown in FIG. 12, the transmission path characteristic of every three sub-carriers can be estimated in the frequency direction for all OFDM symbols.
Next, as shown in FIG. 13, the SP signal interpolated in the time direction is input to an interpolation filter in the frequency direction and undergoes frequency direction interpolation processing through three-times oversampling, whereby the transmission path characteristic of all sub-carriers within the OFDM symbol is estimated. As a result, the transmission path characteristic of all sub-carriers of the received OFDM signal can be estimated.
Since the transmission path characteristic fluctuates depending on time when an OFDM signal is received in motion, the estimation of the transmission path characteristic with an SP signal is difficult. For example, time-direction interpolation processing may be performed on an SP signal when the Doppler frequency, which is the magnitude of the time-dependent fluctuation of the transmission path characteristic, is large. In this case, since the Nyquist condition is not satisfied, a wrong estimation result is obtained for the transmission path characteristic. Then, it is proposed in the past that a Doppler frequency is calculated based on speed information from a speedometer, and the transmission path characteristic is corrected in accordance with the magnitude of the calculated Doppler frequency (see JP-A-10-75226).
However, with the Doppler frequency calculating method by using a speedometer in the past, a Doppler frequency may not be calculated if speed information is not available from the speedometer.
Accordingly, it is desirable to propose a Doppler frequency calculating apparatus and method, which allows the calculation of a Doppler frequency without a speedometer and an OFDM demodulating apparatus that performs equalizing processing by using the calculated Doppler frequency.